1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for determining when a control switch is closed.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically closing the contacts. Most electromagnetic contacts are controlled by switches which place the contactor in the "run" or "start" mode, for example. Typically, a source of alternating voltage or direct voltage is fed through the aforementioned switches to the intelligence portion of the contactor. In those cases where the intelligence portion of the contactor is a microprocessor which reads voltage signals it may be possible under some circumstances due to the effect of capacitance between long lines which are interconnected to the pushbutton switches to provide a false signal, that is, a signal indicative of a closed switch when in fact the switch is open. This will occur during an alternating current operating phase. This is because power supply current for the microprocessor and the control current which is routed in a cable which includes the long lines may be capacitively injected into a parallel cable line which has an open control switch at one end but which has sufficient impedance at the other end to produce a voltage drop due to the presence of the capacitive current. This voltage drop is read as a closed switch by the microprocessor when in fact it should have been read as an open switch. It would be desirable to find a way for the microprocessor to determine whether the voltage it is reading at its input terminals is the result of a true signal or a false signal.